1. Technical Field
Embodiments of the present disclosure relate to an oscillator less affected by process variation and a semiconductor device including the same.
2. Related Art
FIG. 1 is a circuit diagram illustrating a conventional oscillator 101.
The illustrated oscillator 101 is a relaxation oscillator and includes a current source CS, a capacitor C, a comparator CMP, a delay circuit D, an inverter INV, and a PMOS transistor P.
When a value of an output of the comparator CMP is at a high level, because a value of an output of the inverter INV is at a low level, the PMOS transistor P is turned on, so that the capacitor C is charged towards a power supply voltage VDD.
When the charged voltage of the capacitor C becomes equal to or greater than a reference voltage VREF, the value of the output of the comparator CMP becomes a low level. The change in the output of the comparator CMP is delayed by the delay circuit D, and then the output of the inverter INV becomes a high level, at which time the PMOS transistor P is turned off. At this time, the charge charged in the capacitor C is discharged through the current source CS, so that the voltage of the capacitor C is lowered.
When the voltage of the capacitor C falls below the reference voltage VREF, the value of the output of the comparator CMP becomes the high level, and the aforementioned operation is repeated. Accordingly, an output signal Vout of the oscillator 101 becomes a pulse type signal having a cycle time corresponding to the magnitude of the reference voltage VREF and the magnitude of a current of the current source CS.
The current source CS included in the conventional oscillator 101 may, for example, have a structure in which diodes are serially connected to one another. Since such a current source has a characteristic that the size of a current changes according to temperature, the cycle time of the output signal Vout of the oscillator 101 also changes according to the temperature.
FIG. 2 is a graph of a difference according to temperature of signals outputted from the oscillator 101 of FIG. 1.
FIG. 2(a) illustrates a signal at 25° C. for example and FIG. 2(b) illustrates a signal at 90° C. for example.
In the conventional oscillator 101 as described above, since the cycle time of the output signal Vout changes according to temperature, the value of the reference voltage VREF is adjusted such that a constant cycle time can be obtained at a specific temperature, such as 90° C. for example.
However, when process variation occurs, since the size of a current outputted from the current source CS changes according to the process variation, an output signal Vout having an expected cycle time may not be obtained.
FIG. 3 is a graph for explaining the problems of the conventional oscillator.
A line with round markers indicates the case in which process variation is positioned at a Fast-Fast (FF) corner, a line with square markers indicates the case in which the process variation is positioned at a Nominal-Nominal (NN) corner, and a line with triangular markers indicates the case in which the process variation is positioned at a Slow-Slow (SS) corner. As illustrated in FIG. 3, in the conventional oscillator 101, when a temperature is lower than reference temperature (in the example shown in FIG. 3, 90° C.), an increase in a difference among a period (that is, a cycle time) of cycles of the output signal Vout according to the process variation occurs.
When the period of cycles of the output signal Vout of the oscillator 101 changes according to the process variation, as described above, a semiconductor device including the oscillator 101 may operate abnormally. For example, under the assumption that the oscillator 101 controls a self-refresh cycle of a semiconductor memory device, if a signal having a cycle time longer than a preferable refresh cycle is outputted, data may be lost in some cases. In another case, if a signal having a cycle time shorter than the preferable refresh cycle is outputted, then the number of times a refresh is performed may unnecessarily increase, resulting in performance deterioration and power waste of the semiconductor device.